This invention relates to X-ray systems of the ionographic or electron radiographic type and, in particular, to a new and improved cassette or receptor holder, usually referred to as an imaging chamber. In electron radiography, a dielectric sheet or receptor is utilized as the record medium in place of the more conventional photographic film. The dielectric receptor sheet is positioned in the imaging chamber between spaced electrodes, with a gas at high pressure in the gap between the electrodes. The variation in X-ray intensity exiting from the object illuminated by the X-ray source produces a variation in electron density on the dielectric sheet. This electrostatic image on the dielectric sheet is then converted to a visual image using conventional techniques, such as the xerographic process. For further information on the ionographic system reference may be made to U.S. Pat. Nos. 3,774,029 and 3,963,924.
A problem in the design of an electron radiography imaging chamber is containing the gas at high pressure while transmitting X-rays through a large and relatively flat window of the chamber. The X-ray absorption through the window should not only be small, but also should be uniform across the window in order to prevent an X-ray image of the nonuniformities of the window. Generally the inner surface of the X-ray window can be flat or gently curved in one direction or dished in a spherical shape. Preferably, the radii of curvature should be about the same as the distance to the X-ray source. It has been found that in order to keep the X-ray absorption of the window within reasonable bounds, the radiation should pass through no more material than the equivalent of one-half inch of solid beryllium. At the same time, the bulge of the window when subjected to the gas pressure must be kept small. Further, the edge supports for the window must be compatible with a chamber design providing for insertion and removal of the electrostatic image receptor sheet.
Typical picture sizes for medical X-rays are 8 .times. 10 inches and 14 .times. 17 inches, and a simple half-inch thick plate is not strong enough for use as a window at the operating pressure of the chamber, which typically is 10 atmospheres.
It is usually desired that the electrode surfaces at the gap in the imaging chamber have a spherical configuration with the X-ray source as the center. The reasons for this construction are well recognized and a number of imaging chamber configurations have been proposed for achieving this end. U.S. Pat. No. 3,828,192 shows a structure with spherical electrodes with the receptor sheet stretched against the convex surface of one of the electrodes. U.S. Pat. No. 3,963,924 shows an imaging chamber where one or both of the electrodes comprises a conducting layer on the dielectric receptor sheet, with the radiation absorbing gas on one side of the sheet and another gas on the other side of the sheet, with the differential pressure of the gases producing a spherical shape for the sheet. U.S. Pat. Nos. 3,859,529 and 3,883,740 show another approach wherein complex electrode structures are utilized to produce a spherical electric field in a planar gas gap.
These prior art structures suffer from various disadvantages including unsatisfactory radiation window design, requirement for complex electrode structure, and requirement for balancing of two gas supplies with an unsupported receptor sheet. Accordingly, it is an object of the present invention to provide a new and improved imaging chamber with X-ray window which will have the desired radiation transmission characteristics while being structurally secure and readily operable at elevated gas pressures. A further object is to provide a new and improved mechanism for closing and opening the imaging chamber and for positioning the receptor sheet therein. A particular object is to provide a new and improved method and apparatus for conforming the flexible receptor sheet to the previously determined configuration of a rigid electrode. Other objects, advantages, features and results will more fully appear in the course of the following description.